1. Field of the Invention
The present invention relates to a displacement detecting device for detecting displacement of a surface-to-be-measured by a non-contact sensor which uses light emitted from a light source. More particularly, the present invention relates to a technology for detecting vertical displacement of the surface-to-be-measured.
2. Description of the Related Art
Conventionally, a displacement detecting device for measuring displacement and shape of a surface-to-be-measured by light in a non-contact manner is widely used. As a typical example, there is a method in which a laser is irradiated onto the surface-to-be-measured, and change of the position of the reflected light is detected by a PSD. However, the problems with such a method are that it is susceptible to the influence of the tilt of the surface-to-be-measured, that the sensitivity is low, and that if measurement range is widened, the measurement resolution will be degraded.
In contrast to the aforesaid method, there is another method in which a Michelson interferometer is used, wherein the surface-to-be-measured serves as a mirror. With such a method, the measurement range becomes wide and good linearity can be achieved; however, when the measurement range becomes wide, the measurement will be affected by variation in wavelength of the light source and variation in refractive index of the air.
On the other hand, the light emitted from the light source is condensed on the surface-to-be-measured by an objective lens, and the light reflected by the surface-to-be-measured is condensed by an astigmatic optical element and incident on a light receiving element so as to generate a focus error signal by an astigmatic method. Further, by using the focus error signal, a servomechanism is driven to displace the objective lens so that focus position of the objective lens is located on the surface-to-be-measured. At this time, there is a method in which the scale of a linear scale integrally attached to objective lens through a connection member is read to thereby detect the displacement of the surface-to-be-measured (see, for example, Japanese Unexamined Patent Application Publication No. H05-89480). The merits of such a method are that it is less susceptible to the influence of the tilt of the surface-to-be-measured and that it is possible to perform measurement in a wide measurement range with high resolution.
In the displacement detecting device disclosed in Japanese Unexamined Patent Application Publication No. H05-89480, in order to improve the accuracy of the displacement detection, NA (Numerical Aperture) of the objective lens is set large so as to reduce the beam diameter condensed on the surface-to-be-measured. For example, when the beam diameter formed on the surface-to-be-measured is about 2 μm, the detection accuracy of the linear scale will be in a range from several nm to several hundreds nm.